We are currently working on a G protein-coupled receptor activated by extracellular Ca++ (in the millimolar range) with the standard seven transmembrane domain structure but with a unique, approximately 600 amino acid extracellular amino terminal domain (ECD). The latter contains multiple asp and glu residues speculated to be involved in Ca++ binding, 9 putative glycosylation sites, multiple cysteines conserved in the related ECD of the metabotropic glutamate receptor family, and a putative signal peptide. Mass spectrometry is being used to characterize both the intact protein and digested fragments in an effort to help define number, location and nature of glycosylation as well as possible disulfides.